Many different types of filtration apparatus have been used to remove suspended particles from solutions. Conventional filtering equipment, however, frequently becomes ineffective due to a loss of filtering area as well as the subsequent loss of flow rate due to the buildup of particulate matter on the surface of the filter and/or inside the matrix of the filter material. Such a phenomena is particularly true in the filtration of solutions such as blood plasma containing lipid micelles and/or precipitates formed by pH or ionic strength alterations. Such problems also occur when filtering cooking oil to remove impurities such as crumbs, flour particles and carbonized materials that cause or aid the formation of free fatty acids which turns the cooking oil rancid.
Although it is possible to increase surface area of a filter to compensate for loss of filtration area, such increased surface area only contributes to the loss of filtrate due to non-specific adsorption by the filter. Depth filters are frequently used to achieve a higher flow rate or to increase the amount of precipitate retained before the filter is plugged, but with such depth filters the loss of material from the filtrate may be even greater due to absorption by the filter material (as with cellulose type materials) or by retention of solution within the pore spaces themselves. When filtering small volumes of biological solutions, the loss of material in the filtrate often proves to be a serious problem, particularly when the solution to be recovered shows a specific loss of the protein of interest and the protein is a minor constituent of the total.
When filtering plasma containing pH or ionic strength precipitates, the loss of desired proteins may be very high due to the formation of large aggregates of materials which form a layer on the surface of the conventional filters and then further trap proteins other than those that have been precipitated.
Not only do traditional surface or depth filters retain a significant amount of filtrate, but removal and/or recovery of the precipitate from such filters is both difficult and inefficient. To facilitate removal and/or recovery centrifugation may be used instead of filtration, but the cost and time factors are substantial. Furthermore, maintaining a sterile system is a greater problem with centrifugation than with a filtration system.
There are several requirements for filters which are designed to remove precipitates from plasma. Such a filter must possess a small surface area ratio of filter medium to volume of solution to be filtered. The typical filter should also have a minimal holdup volume, a minimum flow rate of 50-100 milliliters per minute with minimal decay of that flow rate over time, and a minimal loss of desired protein in the filter. The filter should be capable of recovering precipitated material and of being sterilized. At this time, there appears to be no filter commercially available at a low cost which allows filtration of relatively small volumes of solution such as blood plasma.
When filtering hot cooking oil, the filter should also have a minimal holdup volume; a minimum flow rate of 3-5 milliliters per minute per square centimeter 50-100 psi differential pressure across the filter element at the operating temperature with a minimal decay of flow rate over time and an ability to filter large amounts of filtrate. Unlike with a plasma filter, allowing loss of small amounts of filtrate, minimizing loss of precipitate or providing a capability of recovering precipitate are not important considerations. In addition, the components of such a filter must be able to withstand temperatures of up to 300.degree.-500.degree. F. without any adverse effect on the filtering capacity of the filter.
It is therefore a principal object of the present invention to provide a filter for filtering solutions containing particulate material or solid particles which are difficult to process using conventional filters due to the nature of the solution.
Another object of the present invention is to provide a filter having a minimal surface area ratio to volume of solution to be filtered and which provides a minimum flow rate of 1.5-3.0 milliliters per minute per square centimeter.
Yet another object of the present invention is to provide a filter which is readiy sterilizable.
Still another object of the present invention is to provide a filter the life of which is extendable without disassembly or other manual intervention.
A further object of the present invention is to provide a filter having a low non-specific adsorption of the solution being filtered.
Yet another object of the present invention is to provide a filter which has the capacity to remove and recover precipitate.
A still further object of the present invention is to provide a reusable, low cost filter which meets the above objects.